Briefly describing the operation of an image sensor, light energy reflected from an object is absorbed by an optoelectronic converter and electrons are freed by the photoelectric effect. The quantity of freed electrons is proportional to the amount of light absorbed. The freed electrons are accumulated in the optoelectronic converter formed over a semiconductor substrate. The electrons form a signal which is read out of the converter. A photo diode may be used as an optoelectronic converter.
An image sensor may include micro lens arrays that collect light and focus it on the photo diode. The micro lens array may be formed by selectively patterning a wiring layer inside the image sensor and by annealing in a reflow process. The annealing in the reflow process may not form the critical dimension of the micro lens uniformly or properly control the thickness and the radius of curvature of the micro lens. Since the selective patterning process is performed after a metal pad is exposed, the surface of the metal pad may be damaged by a strong alkaline developer used in the patterning process.